Electric smelting process for manganese ores



United States Patent ELECTRIC SMELTING PROCESS FOR MANGANESE ORES Robert T. C. Rasmussen, Ottawa, Ontario, Canada No Drawing. Application February 10, 1953, Serial No. 336,212

3 Claims. (CI. 75-10) V (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may benmanufactured and used by or for the Government'of the United States for governmental purposes without the payment tome of any royalty thereon in accordance with theprovi'sions continuous process wherein unfused charge mixture is maintained over the surface of a molten bath. While the invention is particularly applicable to operation in a topfed electric furnace, and is described 'below'in connection therewith, it may also be utilized in the operation of fuel-fired and other types of furnaces.

The process of this invention may be applied in the production of standard grades'of ferro-manganese and also of silico-manganese from all grades of manganeseore including those materials having a-much lower ratioof Mn to Si than the to 1 ratio of present specifications for metallurgical grade manganese ore. It is particularly useful in the smelting of manganese silicate ores heretofore consideredunsuitable forsmelting operations.

T heusual specifications for metallurgical grade manganese ore havebeen-based ona minimum Mn content of '48 percent; Fe, max. 7 percent; SiOz, max. 10 percent; and P max. 0.18 percent. Manganese silicate materials, such as rhodonite, have not been considered a source=of metallurgical manganese 'raw material because of their high silica content, which may run as high as-45 percent. Except for the Montana-carbonate ores, mostdomestic manganese-bearing raw materials have proved .tobe refractory to mechanical beneficiation to produceconcentrates meeting the specifications of metallurgical grade ore. In most instances wherespecifications can be met, therecovery of manganese is prohibitively low. On the other hand, many of the domestic manganese materials can be upgraded to some extent by beneficiation with acceptably high manganese recovery, but not sufliciently to meet specifications for mangenese content.

The maximum silica specification has been imposed on metallurgical manganese ores because, with the limitations of the smelting operations, silica present in the charges of manganese results in a loss of manganese to the slag that is proportionate to the silica content of the charge. It is also reported that high-silica in thesmelter charge results in higher than'desired silica content in the ferromanganese product and inexcessive loss of manganese by vaporization.

Accordingly, an object of this invention is to provide a continuous process for smelting manganese silicates and other siliceous manganese ores to produce ferroand types.

lice Another object of the invention is to provide a continuous process for manganese smelting which permits utilization of manganese raw materials of higher than presently specified maximum silica content in the production of standard grades of ferro-manganese.

Another object of this invention is to provide a continuous process for selectively smelting manganese from manganese silicate and other siliceous manganese ores.

Another object of the invention is to provide a vcontinuous processfor smelting finely divided -manganese ores and concentrates without prior agglomeration of the charge materials.

' Another object of the invention is to provide a method for controlling the rate at which the charge descends through the furnace and for controlling the'smelting zone temperature in a continuous electric furnace process for smelting manganese materials.

These and other objects and advantages are accomplished by the process of this invention which comprises the continuous, dry-top, electric smelting of manganesebearing raw material mixed with a suitable flux, if required, and with carbonaceous reductant containing at least the stoichiometric carbon requirement for reduction of the desired quantities of Mn, Fe, and -Si, wherein at H least a portion of the total carbon in the charge is supplied as a bulky form of carbonaceous material of low fixed carbon content whereby the rate of descent of the charge through the smelting furnace is regulated and the necessary high temperature for reduction is attained and maintained in the smelting zone. The charge mixture is fed to the furnace more or less continuously to maintain an appreciable depth of loose, unfused charge over the molten bath, and the molten alloy product is tapped from the furnace periodically. The bulky form of carbonaceous material serves the function of retarding the rate at which the ore and flux descend into the smelting zone for a given power input, thereby increasing the proportion of energy input that is available to heatthe smelting zone with the consequent increase-of-temperature in the smelting zone. The bulky carbonaceous material also serves the additional functions of providing a deep, porous and 'highlyinsulating cover over the molten bath to eliminate or lower vaporization loss of metal and to confine the heat to the smelting zone, as well as to permit the use of finely-divided ore and flux without op'erating'difliculties normally attendant upon the use of finely-divided smelting charge materials.

It now becomes apparent that the reason manganese silicates and siliceous manganese ores have not been successfully smelted by methods heretofore employed is because MnSiOs has a melting temperature of 1325 C. whereas the temperature necessary for reduction of this compound is much higher, for example, above about 1500 C. In the smelting of ores containing appreciable MnSiOs or silica the necessary reduction temperature is not attainable by present industrial smelting practice. In such processes, the ore descends into the smelting zone substantially as fast as it is melted at the electrode tips and the temperature of this zone is the fusion or melt ing temperature 'of the ore or mixture of oreand flux entering the smelting zone. Consequently the manganese present as silicate is lost to the slag, and it is probable that high silicacontent results in such a low-"melting charge that even manganese in excess of that combined with silica is not reduced. By retarding the rate at which the charge descends into the smelting zone by using a bulky carbonaceous reductant in the charge to provide a'cha'rge of low density, in accordance with the present invention, it is possible to obtain temperatures inthe smelting zone well above themelting temperature of the charge con= stituents and within the necessary reduction "range.

3 Temperatures of 1500 C. to 1700 C. may be obtained by this method in the smelting of manganese ores.

The bulky form of carbonaceous material employed to supply all or at least a substantial portion of the carbon reductant in the process of this invention may be in the form of wood waste such as sawdust, wood chips, coarseground wood waste known to the timber industry as hogfuel, or a combination of these materials. Hog-fuel or similar coarse material of low density and low fixed carbon content is preferred because such material substantially eliminates dusting even with finely ground ores. Other bulky carbonaceous material such as corncobs, nutshells, fruit pits, peat, and lignite may be used to supply the bulky form of carbon in the smelter charge. The high bulk of the wood waste employed in the process of the invention in comparison with the usual reductants em ployed in smelting operations is illustrated by the following tabulations:

From this tabulation, it is apparent that 6.2 cubic feet of wood waste are required to supply the same quantity of carbon as one cubic foot of wood charcoal, and that 19.3 cubic feet of Wood waste are required to supply the same quantity of carbon as one cubic foot of petroleum coke.

The volume of a charge can be controlled to some degree by controlling the density and particle size distribution of the ore, or sinter, and flux. However the range of volume obtainable by such control is small as compared with the use of wood waste or other bulky form of carbon to supply all or part of the carbon required in the charge. When the charge volume is controlled by use of bulky reductant, as in the present invention, sintering or agglomeration of the ore constituents is unnecessary. However, use of these expedients in combination with the improved means of control is within the scope of the invention.

The smelting of manganese ores may be conducted by the process of this invention either with or without addition of a basic flux to the charge. CaO, MgO, limestone, and similar materials containing basic oxides that combine at high temperatures, with SiOz to form metasilicates are suitable as flux materials. The principal function of the flux is to combine with free SiOz of the charge or to replace M110 in any MnSiO3 compound in the charge, thereby facilitating reduction of Mn and inhibiting reduction of $102 in excess of the quantity it is necessary to reduce to obtain the desired silicon content in the resulting silicomanganese, ferromanganese, or manganese product. In the smelting of manganese silicate ore to produce silico-manganese the process of this invention permits the attainment of a high enough smelting temperature for substantially complete reduction of Mn and reduction of the desired quantity of Si, while maintaining adequate dry-top cover over the charge to avoid an appreciable loss of either metal by vaporization. An excess of thestoichiometric quantity of total carbon for the desired reduction of Mn and Si is used in this operation and all, or substantially all, of the carbon is supplied as a bulky carbonaceous material of low fixed carbon content, such as hog fuel. By using lesser amounts of total carbon, but at least that amount stoichiometrically required for the MnO, manganese can be selectively reduced to a considerable degree as compared with silicon even though not enough basic constituent is present for all of the SiOz remaining in the slag in excess of that present as MnSiOs to be combined as metasilicate. With enough basic constituent available to combine as metasilicate with all of the SiOz in the charge over and above the quantity it is desired to reduce and with proper proportions of total carbon and of carbon in bulky form, a high percentage of the manganese in the charge, up to and more than percent, can be reduced to metal while reducing only a small percentage, such as 30 percent or less, of the total SiOz to Si. The process therefore permits selective reduction of manganese from manganese silicate ores as well as providing for production of silicomanganese of various grades.

The invention will be further illustrated by the following examples of practice but it is not intended that it be limited thereto:

Example 1 The process of the invention was carried out in a continuous, open-top smelting operation of three days dura tion. The furnace was a three-phase, top electrode electric furnace with three three-inch graphite electrodes arranged at the points of an equilateral triangle and with a carbon lined furnace body of approximately 31 inch diameter inside the refractory walls. An Oregon rhodonite analyzing 26.5 percent Mn, 45.3 percent SiOz, 1.48 percent Fe, 2.1 percent A1203, 4.1 percent CaO and 1.6 percent MgO was used in this operation. The furnace was started with charge proportions of 23 pounds of limestone and 126 pounds of hog fuel per 100 pounds of rhodonite. This represented substantially 105 percent of the stoichiometric carbon requirement to reduce all of the manganese and iron and about 36 percent of the silica contained in the rhodonite and provided about 56 percent of the stoichiometric amount of basic oxide required to combine as mctasilicate with the remaining 64 percent of the silica in the rhodonite. After approximately 6 hours operation, the hog fuel proportion was increased to 146 pounds and kept constant at that figure for the balance of the test. After about 32 hours from the start of the test, the limestone proportion was increased to 30 pounds and then was increased at two intervals thertafter to 36 pounds and 46 pounds. The average tapping temeprature by optical pyrometer, Without emissivity correction was 1556 C.

The charge that gave optimum results had a volume of 8.4 cu. ft. per hundred pounds of rhodonite plus limestone, whereas the volume would be 1.3 cu. ft. with the use of metallurgical coke to supply the same quantity of carbon. The following tabulation shows data for each period calculated on the basis of charge containing 100 pounds of rhodonite and with calculated weight of slag produced per 100 pounds of rhodonite.

Period First Second Third Fourth Fifth Charged per 100 lb. rhodonite, 1b.:

Limestone. 23 23 3 36 46 Hog fuel 126 146 146 146 146 Calculated weight per 100 lb.

rhodonite, 1b.:

71. 5 71. 5 70. 4 71. 5 74. 8 5. 32 5.07 4. 89 4. 4. 80 Si 19.0 21. 2 20. 9 20. 2 16.6 Calculated Mn in slag per lb. rhodonite, 1b 6. 61 4. 03 3. 61 3. 64 2. 40 Calculated distribution between products, percent metal:

74. 9 83. 9 85.0 84. 5 90. 5 26. 7 33. 1 32. 9 29.4 27. 0 Mn.S1 ratio in metal 3. 76 3. 37 3. 37 3. 54 4.53 Percent of S10: in slag combined as CaSiOa and MgSiO; 56.8 68.5 80.4 86.7 107. 5

' From the data of the first period of operation it may be noted that the use of hog fuel to supply slightly in excess of the stoichiometric amount of the carbon required to reduce the manganese and iron oxides and 36 percent of the silica and addition of only enough limestone to provide total basic oxide amounting to slightly more than one-half the stoichiometric amount required to combine as metasilicate with the remaining 64 percent of silica resulted in reduction of 74.9 percent of the total manganese and 26.7 percent of the'total silica of the charge to produce a metal product analyzing 71.5 percent Mn and 19.0 percent Si. A further increase in carbon above the stoichiometric amount, as shown in the data of period 2, resulted in an increased reduction of both Mn and Si but lowered the MnzSi ratio in the metal product. By then increasing the amount of flux, as shown in the third, fourth and fifth periods of operation, the reduction of Mn with respect to Si was increased. In the fifth period of operation use of 122 percent of the required stoichiometric amount of carbon to reduce the manganese and iron oxides and 36 percent of the silica contained in the rhodonite and addition of limestone to provide total basic oxide in the charge amounting to 113 percent of the stoichiometric amount required to combine as metasilicate with the remaining 64 percent of the silica resulted in recovery of manganese in the metal product of 90.5 percent as compared with 27.0% for SiOz, with production of a metal product containing 74.8% Mn and 16.5% Si.

Distribution between products, percent in alloy:

Silica I Example 2 A second silicomanganese campaign, was made on a new 48-ton lot of rhodonite from the same deposit, analyzing 23.1 percent Mn, 2.06 percent Fe, and 41.2 percent SiOz. The object here was to hold the proportions of limestone and total carbon in the charge constant while varying the proportion of total carbon supplied as hog fuel from 100 percent to 40 percent. With decreased hog fuel proportion, the tapping temperature dropped from about 1600 to 1450 C., manganese recovery dropped, and power consumed per ton of silicomanganese increased from 7,100 to 10,080 kw.-hr. Overall manganese recovery was much poorer than in Example 1 because of the smaller proportion of hog fuel to supply the total carbon requirement for the lower grade ore. The charge with all carbon supplied as hog fuel had a volume of only 5.6cu. ft. per 100 lb. of rhodonite plus limestone as compared with 8.4 cu. ft. in Example 1.

Starting with 100 percent of the carbon as hog fuel, theproportion was lowered successively to 80, 60 and 40 percent, the balance of the carbon being supplied as Coos Bay coal. Data for the four periods of operation are tabulated below.

Analysis, percent Kw.-hr. per ton Weight Percent of Mn dist. Tapping ratio,

total 0 as Metal to metal, temp Slag/ hog fuel s lgg, percent 0. metal Ore Metal Mn S1 Fe Since the increase in flux to combine with the SiOa as CaSiOg, in efiect, amounted to increasing the excess of carbon over the stoichiometric proportion with every increase in limestone, more carbon was used in the final periods than would be necessary for optimum selective production of Mn metal. The measure of the selectivity with which manganese was reduced in preference to silicon is further illustrated by comparing the Mn to Si ratio in the product. This ratio varied from 3.37 to 1 in the second and third periods to a high of 4.53 to 1 in the fifth period, whereas the initial ore had a manganese to silicon ratio of only 1.25 to 1.

The data for the entire test are tabulated as follows:

Materials charged, 1b.:

From these data the marked trend to lower smelting temperature, higher throughput rate, lower manganese recovery, and higher power consumption per ton of metal produced with decreasing proportion of hog fuel in the charge is unmistakable. The fact the ratio of Mn to Si in the metal remained quite constant while the Fe content increased with decreasing Mn recovery demonstrates the importance of high smelting temperature in reducing MnSiOs. The overall recovery of manganese as silicomanganese from the ore smelted was only 52.3 percent, with 95.5 percent, of the manganese in the ore accounted for in the metal and slag products. This again demonstrates the importance of the proportion of hog fuel and consequent density of the charge. Because of the lower Mn content of this lot of ore, the required carbon for reduction as hog fuel amounted to only 104 pounds per pounds of ore as compared with 146 pounds in the test of Example 1 on the higher-grade ore.

Operation of the smelting process in accordance with this invention as set forth in the foregoing description and examples, with the use of hog fuel or similar bulky form of carbon to supply all or substantially all of the total, carbon in the charge provides smooth, trouble-free continuous smelting conditions in the furnace. The charge mixture needs only to be fed to the furnace at the proper rate to maintain the desired charge level while the product alloy is tapped periodically. The charge descends through the furnace at a rate necessary to enable the required high smelting temperature to be attained whereas vaporization losses are substantially eliminated, with the results of increasing manganese recovery in smelting operations on high grade ore and permitting the process to be carried out on silicate and other low grade ores which heretofore were unsuitable for smelting operations. By regulation of the excess of total carbon above the stoichiometric' requirement for reduction of the manganese compounds, and' with the use of basic flux to combine with the 'SiOzof the charge it"is possible to smelt highly siliceous iron and" manganese bearing raw materials to produce ferromanganese with the reduction and recovery of substantially all of the iron and manganese in the charge while reducing and recovering less than 25 percent of the silica in the charge; The applicability of the process to the smelting and utilization of domestic manganese ores and the economic benefits to be derived therefrom are readily apparent.

It will be appreciated from a reading of the foregoing specification that the invention herein described is susceptible of various changes and modifications without departing from the spirit and scope thereof.

What is claimed is:

1. In a process of continuously producing silicomanganese in an electric furnace by reduction with carbon of raw material-mixed with basic flux, the raw material being manganese silicate plus free silica, in which a mixture of said raw material, basic flux and carbon is fed onto and continuously maintained upon a smelting zone, heat is continuously applied to the mixture in the smelting zone, and molten product is tapped from the furnace, said mixture of raw material and basic-flux having a melting temperature below the reduction temperature of the raw material, the improvement which comprises sufficiently diluting the mixture of raw material and basic flux with bulky carbonaceous material to retard the rate at which the raw material and flux enters the smelting zone so that the temperature of the smelting zone is increased to the required reduction temperature for the raw material, saiddilution being about. 8.4 cubic feet of said carbonaceous material per 100 pounds of raw material and flux whereby a high degree of reduction of manganese with respect to silica isobtained.

2. In a processof continuously producing silicomanganese in an electric furnace by reduction with carbon of raw material mixed with basic flux, the raw material being manganese silicate plus free silica, according to claim 1 in which the dilution is slightly in excess of the stoichiometric amount of carbon required to reduce said manganese silicate and sufficient to reduce a portion only of the free silica and enough flux is present to combine with the remainder of the free silica in the charge whereby a high degree of reduction of manganese With respect to silica is obtained. 1

3. In a process according to claim 1 in which said dilution is as a minimum the stoichiometric amount required to reduce said oxygen compounds of manganese plus a substantial amount of the free silica, and such volume of said carbonaceous material containing not more carbon than about 122% of the stoichiometric amount of carbon required to reduce the manganese and any iron oxides present and about 36% of the silica contained in the raw material whereby a high degree of reduction of manganese with respect to silica is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 856,351 Lash June 11, 1907 882,418 Price Mar. 17, 1908 1,565,689 Van Slyke Dec. 15, 1925 2,358,024 Najarian Sept. 12, 1944 2,755,178 Rasmussen July 17, 1956 FOREIGN PATENTS 500,722 Great Britain Feb. 13, 1939 

1. IN A PRECESS OF CONTINUOUSLY PRODUCING SILICOMANGANESE IN AN ELECTRIC FURNACE BY REDUCTION WITH CARBON OF RAW MATERIAL MIXED WITH BASIC FLUX, THE RAW MATERIAL BEING MANGANESE SILICATE PLUS FREE SILICA, IN WHICH A MIXTURE OF SAID RAW MATERIAL, BASIC FLUX AND CARBON IS FED ONTO AND CONTINUOUSLY MAINTAINED UPON A SMELTING ZONE, HEAT IS CONTINUOUSLY APPLIED TO THE MIXTURE IN THE SMELTING ZONE, AND MOLTEN PRODUCT IS TAPPED FROM THE FURNACE, SAID MIXTURE OF RAW MATERIAL AND BASIC FLUX HAV-ING A MELTING TEMPERATURE BELOW THE REDUCTION TEMPERATURE OF THE RAW MATERIAL, THE IMPROVEMENT WHICH COMPRISES SUFFICIENTLY DILUTING THE MIXTURE OF RAW MATERIAL AND BASIC FLUX WITH BULKY CARBONACEOUS MATERIAL TO RETARD THE RATE AT WHICH THE RAW MATERIAL AND FLUX ENTERS THE SMELTING ZONE SO THAT THE TEMPERATURE OF THE SMELTING ZONE IS INCREASED TO THE REQUIRED REDUCTION TEMPERATURE FOR THE RAW MATERIAL, SAID DILUTION BEING ABOUT 8.4 CUBIC MATERIAL AND FLUX WHEREBY A HIGH DEGREE OF REDUCTION OF MATERIAL AND FLUX WHEREBY A HIGH DEGREE OF REDUCTION OF MAGANESE WITH RESPECT TO SILICA IS OBTAINED. 